app: don't potentially overread buffer

If a file has more than 4kB on the first line and starts with #!, then
we would pass a non-null terminated string to strchr. Found by coverity.

app/plug-in/gimpinterpreterdb.c

@@ -733,7 +733,7 @@ gimp_interpreter_db_resolve (GimpInterpreterDB  *db,
     return resolve_extension (db, program_path);
 
   memset (buffer, 0, sizeof (buffer));
-  len = read (fd, buffer, sizeof (buffer));
+  len = read (fd, buffer, sizeof (buffer) - 1); /* leave one nul at the end */
   close (fd);
 
   if (len <= 0)